You can change the output of the invitation email. (OssnInvite email sent by user to his friends to join the network).

<?php
function my_component_init(){
		ossn_add_hook('invite', 'user:options', 'my_component_hook');
}
function my_component_hook($hook, $type, $return, $params){
		//$params['email' 'user' 'subject' 'message' 'actual_message' ] 
		//$params['user'] contain user instance who is sending invitation
		return $widgets;
}
ossn_register_callback('ossn', 'init', 'my_component_init');